1. Field of the Invention
The present invention relates to the technology of an optical storage medium, and more particularly, to an on-line output power calibrating method for making an optimum power calibration of an optical pick-up precise and increasing the stability of the optimum power calibration.
2. Related Art
In the present optical recording technology, an optimum power calibration (OPC) is usually performed before writing data into a disc in order to reduce failure of writing and make the written data more stable. By performing the optimum power calibration procedure, one set of optimum laser power suitable for writing data in this optical disc can be decided, such that the jitter value of the radio frequency (RF) data marked on the optical disc after writing or the decoding error rate can be effectively suppressed and the optimum writing quality can be achieved.
In the DVD format Book, the optimum power control procedure is defined according to a β method and a γ method. The β method is mainly applied to a one-time recordable optical disc, while the γ method is mainly applied to a rewritable optical disc. In these two methods, 15 stages of laser beams with different power are mainly utilized to perform the test writing on the optical disc to select the optimum write power. However, the optical pick-up ages with the elapse of used time, thereby causing the output power to become inaccurate, which deteriorates the optimum write power calibration result. In order to explain this phenomenon, illustrations will be made with reference to the circuit structure of the optical pick-up.
FIG. 1 shows the circuit architecture of a conventional optical pick-up. Referring to FIG. 1, the circuit includes a digital-to-analog converter 101, a laser pick-up diode driving circuit 102, a laser pick-up diode 103, a beam splitter 104, a focus lens 105, a front photo diode 106, an optical storage medium 107 and an eight-beam splitter diode 108. The laser pick-up diode driving circuit 102 has three analog input channels DG1 to DG3, each of which corresponds to the digital-to-analog converter 101 and one of enable inputs EN1 to EN3. Generally speaking, a power level outputted from the laser pick-up diode 103 is controlled according to input digital values DG1 to DG3 of the digital-to-analog converters 101, wherein DG1, DG2 and DG3 respectively correspond to P1, P2 and P3. In addition, the enable input signals EN1 to EN3 control the times when the laser pick-up diode 103 outputs the power (P1, P2, P3). Transfer functions between each of the channel inputs DG1 to DG3 to the laser power output have different weightings. Generally speaking, the power that can be controlled by the input digital value DG1 is higher, so its weighting is larger; the power that can be controlled by the input digital value DG2 is lower, so its weighting is smaller; and the main function of the input digital value DG3 is to cool the laser pick-up. Because the laser pick-up diode 103 cannot output too much power for a long time, it needs to be cooled for a period of time to prevent the laser pick-up diode 103 from burning out. Because the input digital value DG3 is less associated with the output power of the laser pick-up diode, discussions thereof will be omitted.
In the prior art, the input digital value DG1 corresponding to the analog input channel with the maximum weighting is under the close loop control. That is, the finally outputted power P1 of the laser pick-up diode 103 detected by the front photo diode 106 is fed back through the current IFPD measured by the front photo diode 106. The input digital value DG1 is adaptively adjusted according to this feedback mechanism such that the laser pick-up diode 103 outputs the predetermined target power. However, the input digital value DG2 corresponding to the analog input channel with the smaller weighting is under the open loop control. In other words, the digital value DG2 corresponding to the power P2 outputted therefrom is found according to a look-up table. Because the control mechanism of the output power P2 pertains to the open loop control, the manufacturer has to perform a ship-out calibration on this channel to create the look-up table of the output power P2 before the optical drive is shipped out.
Precisely speaking, the output power of the laser pick-up diode 103 is determined by adding the output power P1 to the output power P2. That is, the output power of the laser pick-up diode 103 is determined by the input digital values DG1 and DG2. For example, because the output power P1 is under the close loop control, the output power of the laser pick-up diode 103 may be precisely determined as 4 mW, 8 mW, 12 mW and 16 mW according to the input digital value DG1. However, when the output power has to reach 5 mW, the output power P1 makes the laser pick-up output 4 mW by the close loop control. Then, the digital value DG2 is looked up through the look-up table of the output power P2 and then outputted to the corresponding analog-to-digital converter 101.
However, with the aging of the laser pick-up diode 103 or the unstable power supplied from the computer to the recorder, the input digital value DG2 looked up from the look-up table of the output power P2 becomes inaccurate. For example, the original power to be outputted is 5 mW, the laser pick-up diode 103 only can output 4.3 mW according to the default input digital value DG2 because of the aging of the laser pick-up diode 103 or because of the unstable power. This condition makes the optimum power calibration (OPC) become inaccurate, thereby causing the poor writing quality or writing failure.